As the portable power source for laptop computers, mobile phones, digital cameras and the like, there is an increasing demand for lithium ion secondary batteries featuring a high energy density. A focus is also directed to lithium ion secondary batteries as the power source for electric automobiles which are desired to reach a practical level because of environment friendliness.
Conventional lithium ion secondary batteries use carbonaceous materials as the negative electrode active material. To meet the recent demand for higher capacities, it is envisioned that silicon and other metals capable of alloying with lithium and oxides thereof which are expected to provide a high charge/discharge capacity are used as the negative electrode active material. The use of alloying metals as the active material is expected to provide a high capacity, but can cause an irreversible phenomenon that once lithium in the positive electrode material is introduced into the negative electrode material during the first charging step, not all lithium ions are taken out during subsequent discharge, with a certain amount being left fixed within the negative electrode. This undesirably results in a battery having a decreased discharge capacity and a degraded capability.
One solution proposed for solving the problem is to previously incorporate a lithium source in a negative electrode material. The lithium source may take various forms including metallic lithium powder (JP-A 5-67468 or U.S. Pat. No. 5,162,176), metallic lithium foils (JP-A 11-86847, JP-A 2004-303597, JP-A 2005-85508), and lithium compounds (Japan Pat. 3287376 and JP-A 9-283181).
These approaches, however, are industrially unacceptable because the manufacture process lacks safety and the operation in an atmosphere where lithium remains non-reactive is cumbersome.